I experience stiffness in the morning when I indulge in to much of the softer cheeses's. I lay off those and I do fine. Sounds like you may have introduced something you are not aware of into your diet.
As for medical opinions the head rheumatoligists at the University of Utah is the one that stated I was in remission due to my diet and recommended no medications. I was told about the benefit of this diet by the first Doctor who diagnosed me with AS. And finally my personal Doctor after two years of my diet and progress which is checked every year speaks highly of it. He has told more than one patient about it. Also my allergist whom I used to go to four times a year before I started the diet has called and asked me to talk to some of his patients about it.
I don't know that it will work for everyone, I do know it works for me. Proven by medical tests, scans, blood work etc. It's not an easy road but for me it is the best road. I went from not being able to walk 100 yards to riding my mules and hiking mountain tops and canyons.
So good luck I hope you get it figured out.
ETTE.
Darrel